The present invention relates to methods and compositions for use in subterranean operations. More particularly, the present invention relates to methods that use degradable surfactants for generating acids in a subterranean formation.
Surfactants may be used in a variety of subterranean operations. Such operations include, but are not limited to, drilling operations, stimulation treatments (e.g., fracturing treatments, acidizing treatments), and completion operations (e.g., sand control treatments). In these subterranean operations, surfactants may be used for a number of purposes, such as emulsifying agents, non-emulsifying agents, foaming agents, defoaming agents, viscosifying (e.g., gelling) agents, dispersants, wetting agents, and the like.
While a variety of surfactants have been used in subterranean operations, various problems have been associated with their use. For instance, certain surfactants used heretofore may have undesirable environmental characteristics and/or may be limited by strict environmental regulations in certain areas of the world. As a result, use of these surfactants in subterranean operations may result in the potential for the bioaccumulation and/or persistence of such surfactants in the environment, which may create potential dangers for their use, such as adverse effects on shrimp and other aqueous aquatic species. Additionally, certain surfactants may tend to oil wet a surface which may be undesirable in certain instances.
Degradable materials also are commonly employed in subterranean operations. In some instances, the degradable materials may be capable of releasing a desirable degradation product, e.g., an acid, during its hydrolysis. The acid released by certain degradable materials may be used to facilitate a reduction in viscosity of a fluid or to degrade a filter cake, as well as for numerous other functions in subterranean operations. However, degradable materials used heretofore, such as particulate poly(lactic) acid may not be water-soluble.
A variety of viscosified treatment fluids may be used in subterranean operations, such as drilling fluids, fracturing fluids, and gravel-pack fluids. Oftentimes, after the viscosified fluid has performed its desired task, it may be desirable to reduce its viscosity so that the treatment fluid may be recovered from the formation and/or particulate matter may be deposited within the formation at a desired location. Reducing the viscosity of a viscosified treatment fluid is often referred to as “breaking” the fluid.
For some viscosified treatment fluids, viscosity may be related to pH. Thus, viscosity-reducing agents that reduce the pH of the treatment fluid may be added to reduce the viscosity of the fluid. Internal breakers, such as enzymes, oxidizers, acids, or temperature-activated viscosity reducers, also may be used to reduce the viscosity of viscosified treatment fluids. Unfortunately, these traditional breakers may result in an incomplete or premature viscosity reduction. Premature viscosity reduction is undesirable as it may lead, inter alia, to particulates settling out of a fracturing or gravel-packing fluid in an undesirable location and/or at an undesirable time. Moreover, conventional non-delayed breakers begin to reduce the viscosity of the viscosified fluid upon addition and continue to reduce the fluid's viscosity with time until the fluid is completely broken or until the breaker is expended. Since the breaking activity begins immediately, it is common practice to start with excess gelling agent to offset the point at which the viscosity falls below an acceptable level. Using excess gelling agent is not only an added expense; it also may lead to excessive friction pressure during treatment placement and/or in additional formation damage.
As an alternative to using traditional breakers, breaking a viscosified treatment fluid also may be accomplished using just time and/or temperature. For example, the viscosity of most treatment fluids will reduce naturally if given enough time at a sufficient temperature. However, such methods generally are not practical as it is generally desirable to return the well back to production as quickly as possible as opposed to waiting for the viscosity of a treatment fluid to naturally decrease over time.
Inclusion of an acid-releasing degradable material in a viscosified treatment fluid, such as a gelled (and optionally crosslinked) treatment fluid, may be used to facilitate a reduction in viscosity of such fluid. Generally, these degradable materials likely will hydrolyze over time due to contact with water present in the fluid, thereby releasing an acid. Upon its release, the acid may function, inter alia, to reduce the viscosity of the viscosified treatment fluid, for example, by breaking the crosslinks in the treatment fluid, reducing the pH of the treatment fluid sufficiently to reverse the crosslinks therein, and/or breaking down the backbone of the gelling agent present in the treatment fluid. In some instances, the acid released by the degradable materials may breakdown gelling agents at temperatures above about 150° F.
Degradable materials capable of releasing an acid also may be used in the degradation of acid-soluble materials present in a subterranean formation, such as the formation itself (e.g., in matrix acidizing, fracture acidizing, etc.), calcium carbonate, and acid-soluble components of completion equipment (such as plugs, sleeves, etc.). In some instances, the acid released by the degradable material may be used to facilitate the setting of an acid-settable resin. In some instances, the acid may activate an oxidizer (e.g., sodium chlorite) that is a stronger oxidizer at a lower pH. In other instances, filter cakes commonly may be formed by a fluid (e.g., a drill-in and servicing fluid) on the face of a portion of a subterranean formation, inter alia, to minimize damage to the permeability thereof. The filter cake often comprises an acid-soluble component (e.g., a calcium carbonate bridging agent) and a polymeric component (e.g., starch and xanthan). Before desirable fluids, such as hydrocarbons, may be produced, the filter cake generally is removed. To facilitate the degradation of the acid-soluble component, a degradable material capable of releasing an acid may be utilized. Filter cakes also may be removed using an acid where the filter cake does not contain an acid-soluble component, for example, by degrading the underlying carbonate adjacent, if the filter cake is present in a carbonate formation. In some instance, to facilitate the degradation thereof, the filter cake may be contacted by a treatment fluid that comprises water and the degradable material. The acid produced by the resultant hydrolysis of the degradable material may interact with the acid-soluble component of the filter cake and/or the underlying carbonate adjacent to the filter cake in such a way as to facilitate their degradation. Among other components, the treatment fluid may contain oxidizing agents or enzymes suitable to facilitate the degradation of the polymeric component of the filter cake.